Light-emitting diodes (LEDs) belong to a class of the most efficient light sources among currently available light sources. In particular, white LEDs find a rapidly expanding share in the market as the next-generation light source to replace incandescent lamps, fluorescent lamps, cold cathode fluorescent lamps (CCFL) for backlight, and halogen lamps. As one configuration for white LED, a white LED device (LED lighting device) constructed by combining a blue light-emitting diode (blue LED) with a phosphor capable of emitting light of longer wavelength, for example, yellow or green light upon blue light excitation is implemented on a commercial basis.
The mainstream of the white LED structure is a system in which a phosphor in admixture with resin or glass is placed on or near a blue LED so that the phosphor layer substantially integrated with the blue LED may convert the wavelength of part or all of blue light to produce pseudo-white light, to be called white LED element system. Also some light-emitting devices are based on a system in which a phosphor is spaced apart from a blue LED by a distance of several millimeters to several tens of millimeters so that the phosphor may cause wavelength conversion to part or all of blue light. Particularly when the phosphor tends to degrade its properties by the heat generated by LED, the far distance of phosphor from the LED is effective for improving the efficiency of light-emitting device and suppressing the variation of color tone. A phosphor-containing wavelength conversion member to be spaced apart from an LED light source is known as remote phosphor plate, and such a light emitting system is known as “remote phosphor technology.” Recently active efforts are made on the light emitting system of remote phosphor technology because an improvement in overall color variation and other improvements are advantageous when the system is used for illumination.
The light-emitting device of remote phosphor technology is generally constructed, for example, by placing a wavelength conversion member, which is made of resin or glass having yellow light-emitting phosphor (referred to as yellow phosphor, hereinafter) particles, green light-emitting phosphor (referred to as green phosphor, hereinafter) particles or red light-emitting phosphor (referred to as red phosphor, hereinafter) particles dispersed therein, forward of a blue LED as the remote phosphor, to provide a light emitting device wherein yellow fluorescence of center wavelength around 570 nm is emitted in response to incident blue light of wavelength around 450 nm and combined with light emitted by the blue LED and transmitted by the remote phosphor. Examples of the phosphor used as the remote phosphor include Y3Al5O12:Ce, (Y,Gd)3(Al,Ga)5O12:Ce, (Y,Gd)3Al5O12:Ce, Tb3Al5O12:Ce, CaGa2S4:Eu, (Sr,Ca,Ba)2SiO4:Eu, and Ca-α-SiAlON:Eu. Also, sulfide-based phosphors which are normally difficult to use on chips are generally used.
However, the light-emitting device of remote phosphor technology is constructed such that the wavelength conversion member containing yellow or green phosphor particles is disposed in a region where the contour of the light-emitting device is seen. The remote phosphor plate looking yellow as the outer appearance in the non-emissive state is often mounted in such a state that the plate may be seen from the outside, substantially detracting from the esthetic appearance of the light-emitting device in the unlit state. Thus for the prior art light-emitting devices, particularly in the application where outer appearance is important, an attempt is made to improve their outer appearance by enclosing the device in a white lamp shade with low transparency, but instead a lowering of emission efficiency is inevitable. While it is desirable in consideration of emission efficiency to omit such a lamp shade or cover, the resultant loss of the esthetic appearance in the unlit state becomes a dilemma.